1. Field of the Invention
The invention relates to an exhaust-gas cleaning device for an internal combustion engine, in particular for a stationary gas engine, with at least one catalytic converter arrangement, wherein exhaust gas from the internal combustion engine can flow in a flow direction through the catalytic converter arrangement, and comprises, along the flow direction, a plurality of catalytically active regions that are in particular independent of one another, wherein a heat dissipation device is provided which comprises at least one heat absorbing device and a heat sink, wherein the at least one heat absorbing device is provided between at least two catalytically active regions, the at least one heat absorbing device being in contact with the heat sink.
2. Description of Related Art
Modern high-performance lean burn engines, in particular stationary gas engines, have very high power densities and efficiencies with relatively low manufacturing and maintenance costs. For this reason lean gas engine units are among the most economical energy generation technologies on the market. Compared to diesel engines the emissions of pollutants and toxic substances is also very low, in which connection NOx emissions can be reduced to very low values by internal engine measures and CO and formaldehyde emissions by exhaust gas catalytic measures. Natural gas-operated lean engines on the other hand have relatively high emission values in terms of unburned hydrocarbons, in particular methane (CH4). In contrast to CO, conventional exhaust gas cleaning devices can convert only very small amounts of CH4. In order to achieve sufficiently high conversion rates, in addition to special formulations for the active surface of the catalytic converter arrangements of exhaust gas cleaning devices temperatures of at least about 600° C. are also necessary.
Since the exhaust gas temperature of modern highly supercharged lean burn gas engines after an exhaust gas turbocharger is only about 300-400° C., an exhaust gas cleaning device that is effective for CH4 cannot be used after the exhaust gas turbocharger. In order to achieve a corresponding reaction for the conversion of unburned hydrocarbons in an exhaust gas cleaning device of a highly supercharged lean burn gas engine, the exhaust gas cleaning device must therefore be arranged in front of an exhaust gas turbine. In this way it can be ensured that the temperature necessary for this chemical reaction is achieved.
In the conversion of the unburned hydrocarbons part of the chemical energy contained therein is converted by the chemical reaction into heat. Since the reaction rate in the exhaust gas cleaning device is a function of the temperature, the conversion takes place more rapidly and completely the higher the temperature. Therefore as soon as the concentration of unburned hydrocarbons exceeds a certain value, the rise in temperature on account of the reaction leads to an increase in the reaction rate and thus to a self-sustaining process. The temperature continues to increase until all the unburned hydrocarbons contained in the exhaust gas are completely converted in a very short stretch. With correspondingly increased concentrations of unburned hydrocarbons there is consequently a localized thermal overloading of the catalyst support or the carrier material of the catalytic converter arrangements even melts. This effect is further intensified in the case where the exhaust gas cleaning device is installed in front of an exhaust gas turbine, since the exhaust gas is under a pressure of about 3-4 bars and the energy density of the chemical energy contained in the unburned hydrocarbons is therefore correspondingly increased. One of the most serious problems in this connection is the thermal destruction of regions of an exhaust gas cleaning device due to too high concentrations of unburned hydrocarbons, such as occur for example with ignition failures or misfiring. In principle however it is virtually impossible to avoid ignition failures in gas engines.
As a countermeasure against such an intensified corrosion of a catalyst support body on account of highly exothermic reactions caused by unburned hydrocarbons, DE 100 46 278 A1 for example shows a catalyst support body that has two different zones in the flow direction of the exhaust gas, wherein the first zone has a higher surface specific heat capacity than the second zone. In this way a relatively large amount of thermal energy is extracted from the exhaust gas in the first zone, whereby an intensified corrosion in the second zone can be counteracted. The disadvantage in this case however is that the heat absorption takes place through the catalyst support body itself. Since the heat capacity and the thermal conductivity of the catalyst support body are limited, there is therefore only a slight dissipation of the generated heat from the catalytic converter arrangement to the surrounding structure.
Exhaust gas cleaning devices with respectively a catalytic converter arrangement are also known from U.S. Pat. No. 5,474,745 A and EP 1 111 212 A2, which comprise cooling devices for dissipating heat from the catalytic converter arrangement.